Gears employed in printing presses commonly are of one of four types. Either they are keyed to the associated shaft, or, they are a shrink fit on the shaft, or, they are pinned to the shaft, or, they are secured to the associated shaft by one or more machine screws extending radially of the gear radially or of an associated hub of the gear.
In printing presses designed for highly accurate printings in more than one printing ink, such as inks differing in color, composition, or other characteristics, the manner in which the gears are secured to their associated shafts is a limiting factor in the extent of accuracy of the produced prints, this in large measure being dictated by the accuracy of centering of the gears on their associated shafts, and the extent to which a gear is prohibited from moving angularly relative to its supporting shaft.
In the event that the gear is keyed to its associated shaft, then, despite the accuracy of machining of the shaft and the bore of the gear, radial movement of the gear out of absolute concentricity with the shaft can occur, as permitted by manufacturing tolerances in the shaft diameter and that of the gear bore. The gear bore must be of slightly larger diameter than that of the associated shaft in order to permit the gear to be fitted onto the shaft, which in turn permits shifting of the gear in directions radially of the shaft axis.
One manner of eliminating this problem is to shrink the gear onto the shaft by heating the gear, fitting the heated gear onto its associated shaft while in a heated condition, and then permitting the gear to col to ambient temperature and become shrink-fitted onto the shaft.
While such a procedure can be employed with great success, it carries with it the disadvantage that the gear must be reheated before it can be removed from the shaft, a procedure that is totally unacceptable in rotary printing presses, in which the gear sets must be replaced for each diameter of printing cylinder to be employed in that press.
Keyed gears are incapable of adjustment of the angular relationship of the gear relative to the angular position of the printing cylinder. However, unwanted limited angular movement of the gear relative to its associated shaft can occur, this being permitted by the manufacturing tolerances of the key and those of the shaft and gear key ways.
Such keyed gear constructions are undesirable in applications that require reversals in the direction of rotation. In conventional keyed constructions angular slippage will occur on each reversal of the direction of rotation, as permitted by the manufacturing tolerances in the key and keyways.
As an alternative to providing a keyed interconnection between the shaft and the gear, a pinned interconnection can be provided. While such a pinned interconnection can successfully eliminate angular play between the gear and its associated shaft, the result is a permanent interconnection between the gear and the shaft, that prohibits ready removal of the gear from the shaft when required in order to perform an exchange of gears, and thus is unacceptable in printing presses.
Pinning of the gear to its associated shaft does not, however, eliminate play in the radial direction of the gear relative to its associated shaft. Any such play can result in improper meshing of the teeth of the gear with those of the adjacent gears in the gear train, with consequential misalignment, gear markings, and scuffing in the produced prints, and can only be overcome by providing spring-loaded split gears.
Gears secured to their associated shaft by machine screws suffer from the same disadvantages, and are in fact encumbered with further and severe disadvantages.
Due to the existing play between the gear bore and the periphery of the shaft, securement of the gear to the shaft by the use of a machine screw, extending radially of the shaft, such as a grub screw, automatically will move the gear out of concentricity with the shaft at the time the machine screw is tightened down. Further, the machine screw must bite into the outer circumference of the shaft, which then becomes pitted and damaged on each exchange of the gear sets. As before, lack of concentricity of the gear relative to its associated shaft will result in gear chatter, and consequential misalignment, gear markings and scuffings in the finished print. Also, on tightening down of the screw, the screw itself can act to shift the angular position of the gear.
The employment of machine screws to fix the gears to their associated shafts does, however, have one major advantage over the keyed gear construction. It permits adjustment of the angular position of the gear relative to the angular position of the associated shaft and the printing cylinder supported by that shaft, i.e., during set up of the press, the gear can be released from its associated shaft and the cylinder rotated into register with the printings of preceding or succeeding printing cylinders, and, the gear then resecured to its associated shaft with the printing cylinder in the required adjusted position of the angular rotation. However, fitting and damage to the shaft caused by the machine screw reduces the accuracy of such an adjustment.